High-strength Cr-Mo steel and a weld metal thereof used under environment of high temperature and pressure, such as environment in a boiler or a chemical reaction container, are each required to have, at the same time, high strength, high toughness, good creep rupture properties, high SR crack resistance, which ensures no grain boundary cracking during stress relief annealing, and high temper embrittlement resistance, which ensures less embrittlement while in use under high-temperature environment. In particular, material thickness is recently increased along with increased size of equipment, and thus welding heat input gradually increases from the viewpoint of working efficiency. Such increased welding heat input in general coarsens a microstructure of the weld metal, and degrades toughness (or temper embrittlement resistance). Hence, higher toughness and/or temper embrittlement resistance are currently required. Operation condition of such equipment is intentionally increased in temperature and pressure from the viewpoint of high-efficiency operation. The weld metal is also required to be improved in creep rupture properties.
The following approaches have been reported as investigations focusing on toughness and/or temper embrittlement resistances of a weld metal of high-strength Cr—Mo steel.
For example, in PTL 1, a steel sheet composition, a welding material composition, and a welding condition are regulated in detail so that the weld metal has creep strength, toughness, and hydrogen crack resistance at the same time. For example, in PTL 2, a weld metal good in toughness, strength, temper embrittlement resistance, and SR crack resistance is achieved through regulating a composition of each of a wire and bond flux in detail and controlling a welding condition.
For example, in PTL 3, a composition of a weld metal, particularly the content of each impurity element, is controlled to improve toughness, strength, and SR crack resistance of the weld metal. For example, in PTL 4, toughness, strength, and the like of a weld metal is improved by controlling a composition of each of a core wire and a coating material of a welding rod for shielded metal arc welding. For example, in PTL 5, toughness, temper embrittlement resistance, and the like are improved by controlling a composition of each of a wire and bonded flux for submerge arc welding.
For example, in PTL 6, a carbide form at a grain boundary is controlled, and Ostwald ripening of fine carbide particles is inhibited during a creep test, thereby good creep rupture properties of a weld metal are achieved. For example, in PTL 7, it is found that fine Mo2C particles precipitated during temper embrittlement treatment prompt temper embrittlement, and temper embrittlement resistance is improved by suppressing precipitation of such Mo2C particles.